1. Field of the Invention
The present invention relates to a permanent magnet rotating machine, and more particularly, to a permanent magnet rotating machine in which permanent magnets are inserted into magnet insert holes formed in a rotor.
2. Description of the Related Art
Generally, a permanent magnet motor having a rotor in which permanent magnets are inserted into magnet insert holes is used as a motor for driving a compressor which is installed, for example, in an air conditioner or a refrigerator, or as a motor for driving a vehicle or a motor for driving an apparatus installed in a vehicle. Such a permanent magnet motor is typically referred to as an “interior permanent magnet motor (IPM motor)”.
In the permanent magnet motor, generally, a stator has teeth that define slots for holding a stator winding. The rotor is rotatably disposed such that a gap is formed between the outer circumferential surface of the rotor and the teeth top surface. Further, the rotor has main magnetic poles and auxiliary magnetic poles. A magnet insert hole for receiving a permanent magnet is disposed in each of the main magnetic poles. Thus, both of magnet torque produced by magnetic flux of the permanent magnets and reluctance torque produced by the salient pole property of the auxiliary magnetic poles can be utilized.
Conventionally, the outer circumferential surface of the rotor of the permanent magnet motor is circular as viewed in cross section perpendicular to the axial direction of the rotor. In the permanent magnet motor having such a rotor, in some cases, magnetic flux flowing through the teeth may abruptly change when the boundary portions between the main magnetic poles and the auxiliary magnetic poles pass the teeth. The abrupt change of the magnetic flux through the teeth may cause generation of noise and vibration.
Interior permanent magnet motors having a rotor as shown in FIGS. 16 to 19 are provided in order to prevent such abrupt change of the magnetic flux flowing through the teeth. FIGS. 16 and 17 show interior permanent magnet motors of distributed winding type which are disclosed in Japanese laid-open patent publication No. 7-222384. FIGS. 18 and 19 show interior permanent magnet motors of concentrated winding type which are disclosed in Japanese laid-open patent publication No. 2002-78255.
The interior permanent magnet motor shown in FIG. 16 includes a stator 540 having teeth T1 to Tn and a rotor 550. Magnet insert holes 551a to 551d for receiving permanent magnets 552a to 552d are disposed in main magnetic poles of the rotor 550. The outer circumferential surface of the rotor 550 comprises outer circumferential surface portions 550a to 550d each having a circular arc shape as viewed in cross section perpendicular to the axial direction of the rotor. Each of the outer circumferential surface portions has a circular arch shape having a radius R1 and having its center of curvature on a point K. The point K is located on a line (hereinafter referred to as “d-axis”) connecting the center O of the rotor 550 and the center of the assigned main magnetic pole in the circumferential direction and displaced from the center O toward the magnet insert holes 551a to 551d. 
The interior permanent magnet motor shown in FIG. 18 includes a rotor 750 similar to the rotor 550 shown in FIG. 16, and a stator 740 having teeth of wider width than the teeth shown in FIG. 16.
The interior permanent magnet motor shown in FIG. 17 includes a stator 640 having teeth T1 to Tn and a rotor 650. Magnet insert holes 651a1, 651a2 to 651d1, 651d2 for receiving permanent magnets 652a1, 652a2 to 652d1, 652d2 are disposed in main magnetic poles of the rotor 650. The outer circumferential surface of the rotor 550 comprises outer circumferential surface portions 650a to 650d and outer circumferential surface portions 650ab to 650da. The outer circumferential surface portions 650a to 650d each have a circular arc shape having a radius R and having its center of curvature on the point O of the rotor 650. The outer circumferential surface portions 650ab to 650da each intersect with a line (hereinafter referred to as “q-axis”) connecting the center O of the rotor 650 and the center of the assigned auxiliary magnetic pole in the circumferential direction and have a V-shape formed by cutting off associated virtual outer circumferential surface portions (shown by dashed lines in FIG. 17) having the radius R.
The interior permanent magnet motor shown in FIG. 19 includes a rotor 850 similar to the rotor 650 shown in FIG. 17, and a stator 840 having teeth of wider width than the teeth shown in FIG. 17.
In the interior permanent magnet motors shown in FIGS. 16 to 19, each of the rotors is caused to rotate when power is supplied to the stator winding from a power-supply unit such as an inverter.
In the rotor 550 shown in FIG. 16, the distance between the center O of the rotor 550 and the outer circumferential surface of the rotor 550 decreases away from the d-axis of the main magnetic poles. In other words, the distance (gap) between the outer circumferential surface of the rotor 550 and the teeth top surfaces of the teeth T1 to Tn of the stator increases away from the d-axis. Therefore, magnetic flux (magnetic flux X1) is concentrated around the d-axis (on the region facing the teeth T1 in FIG. 16) where the distance between the outer circumferential surface of the rotor 550 and the teeth top surfaces of the teeth is short, so that magnetic saturation is easily caused. When magnetic saturation is caused around the d-axis, magnetic flux flows via the teeth T2, Tn. In this case, the gap between the outer circumferential surface of the rotor 550 and the teeth top surfaces of the teeth T2, Tn is wider than the gap between the outer circumferential surface of the rotor 550 and the teeth top surface of the teeth T1, so that the magnetic flux flowing via the teeth T2, Tn is reduced.
Similarly, in the rotor 750 shown in FIG. 18, magnetic flux is concentrated around the d-axis where the distance between the outer circumferential surface of the rotor 750 and the teeth top surfaces of the teeth is short, so that magnetic saturation is easily caused. When magnetic saturation is caused around the d-axis, magnetic flux flows via the circumferential ends of the teeth top surface of the teeth T1. In this case, the magnetic flux flows via a wider gap between the outer circumferential surface of the rotor 750 and the teeth top surface of the teeth T1, so that the magnetic flux flowing via the teeth T1 is reduced.
When the magnetic flux is reduced, the induced electromotive force of the stator winding decreases. It is necessary to increase the number of turns of the stator winding in order to compensate for the decrease of the induced electromotive force of the stator winding. However, if the number of turns of the stator winding is increased, the copper loss of the stator winding increases, so that the efficiency of the motor is deteriorated.
In the rotor 650 shown in FIG. 17, the outer circumferential surface portions 650a to 650d each have a circular arc shape having a radius R and having its center of curvature on the point O of the rotor 650. Therefore, unlike the rotor 550 shown in FIG. 16, magnetic flux is not concentrated around the d-axis. However, a larger change is caused in the amount of magnetic flux around the boundary portions between the circular arc outer circumferential surface portions 650a to 650d and the outer circumferential surface portions 650ab to 650da cut off into a V-shape. Therefore, harmonic components contained in the waveform of the induced electromotive force of the stator winding increase.
Similarly, in the rotor 850 shown in FIG. 19, a larger change is caused in the amount of magnetic flux around the boundary portions between circular arc outer circumferential surface portions 850a to 850d and outer circumferential surface portions 850ab to 850da cut off into a V-shape. Therefore, harmonic components contained in the waveform of the induced electromotive force of the stator winding increase.
A sensorless control system may be used as a control system of the permanent magnet motor. In this sensorless control system, the position of the rotor is detected by using the input voltage and input current, assuming that the induced electromotive force has a sinusoidal waveform. In the sensorless control system, the accuracy of detecting the rotor position drops off as harmonic components contained in the waveform of the induced electromotive force increase. When the rotor position detecting accuracy drops off, an optimum control cannot be achieved, and thus the motor efficiency is reduced.
In the concentrated winding type, compared with the distributed winding type, the stator winding can be more efficiently held within the slots, and the amount of the stator winding sticking out of the slots is smaller. When the amount of the stator winding sticking out of the slots is smaller, the copper loss of the stator winding is smaller. However, in the concentrated winding type, the length of the teeth end portions (end portions extending from the teeth body in the both circumferential directions) in the circumferential direction is longer than that in the distributed winding type. Therefore, magnetic saturation is more easily caused at the teeth end portions than in the distributed winding type.
In the distributed winding type, compared with the concentrated winding type, a larger amount of the stator winding sticks out of the slots, and thus a larger copper loss of the stator winding is caused. However, in the distributed winding type, a larger number of teeth of the stator face one pole of the rotor than in the concentrated winding type. Therefore, magnetic flux flowing from the teeth of the stator to the rotor or magnetic flux flowing from the rotor to the teeth of the stator is dispersed, so that the magnetic flux is less concentrated on the teeth end portions. Thus, in the distributed winding type, compared with the concentrated winding type, the density difference of the magnetic flux in the teeth end portions can be reduced so that noise and vibration are lower (for example, about 10 dB lower). Further, as for the distributed winding type in which the magnetic flux is less concentrated on the teeth end portions, it is not necessary to consider local demagnetization of permanent magnets. Therefore, the thickness of the permanent magnet in the direction of magnetization can be reduced, and thus the use of the permanent magnet can be reduced.
Either one of the winding types can be selected according to the properties to be required of the apparatus in which the interior permanent magnet motor is installed.
In the interior permanent magnet motor, whether the distributed winding type or the concentrated winding type, if any of the rotors as mentioned above is used, magnetic flux is concentrated in a particular region, or a larger change of the amount of magnetic flux is caused in a particular region, so that the efficiency decreases.
Further, assignee of the present invention developed and filed patent applications for interior permanent magnet motors which are disclosed in Japanese laid-open patent publication Nos. 2004-260972 and 2005-86955. In the interior permanent magnet motors disclosed in Japanese laid-open patent publication Nos. 2004-260972 and 2005-86955, an outer circumferential surface of the rotor comprises first outer circumferential surface portions which intersect with the d-axes of the main magnetic poles and second outer circumferential surface portions which intersect with the q-axes of the auxiliary magnetic poles, as viewed in cross section perpendicular to the axial direction of the rotor. Each of the first and second outer circumferential surface portions has a curve profile which bulges radially outward, and the radius of curvature of the second curve profile is larger than that of the first curve profile.